A firefighter searches for people trapped in mudslide debris on Jan. 10 in Montecito, Calif.

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Search and rescue efforts continue in Santa Barbara County, Calif., after deadly mudslides killed at least 17 people and destroyed dozens of homes.

Flash floods swept down hillsides recently devastated by wildfires, highlighting the heightened risk of mudslides after firefighters contained the flames. Officials say 43 people are still missing, and experts are warning that more rainfall could cause more destruction.

Charles Crail lives in a part of Montecito that was under a voluntary evacuation order, and he was at home when the mudslides happened. He tells Here & Now's Peter O'Dowd the mudslide looked like "a tributary of the Mississippi River" as cars and homes swept up in the debris roared by.

"I've lived there ... since 1972, I've been through other floods. I've never seen that part of Montecito flood like this," he says. "So if you hear of hundred-year events, this was a thousand-year event."

Crail says the mud rose to the edge of his windows, but did not come inside the house. His neighbors' homes were destroyed and the rest of his neighborhood suffered significant damage.

"It's a total miracle that this house didn't just get washed away," he says. "The one right below our house, you wouldn't even know ... it's a house. Every window is gone, all the furniture is gone, everything's gone."

More than 21,000 people evacuated in Santa Barbara and Ventura counties, according to the Associated Press, but flash floods reached areas closer to the ocean where evacuation orders were voluntary. Many people stayed because they had just returned after fleeing the Thomas Fire, which burned over 280,000 acres and destroyed more than 1,000 structures last month.

Hillsides become more susceptible to mudslides after wildfires strip them of vegetation, says Francis Rengers, a geologist with the U.S. Geological Survey, adding that the threat has become much more serious due to the high recurrence of wildfires.

"The key factors that make Southern California really susceptible to this kind of threat is that the mountains are tectonically active, so they're actively moving up," he tells O'Dowd. "That's creating shattered rocks and creating really steep slopes, and the vegetation and the climate in the area are adapted to wildfires that occur on average about once every 30 years."

The size and speed of mudslides mean they sound like a freight train roaring down a mountain, Rengers says.

"They can move 35 to 50 miles per hour, and some of the boulders that we've seen on the ground are the size of vehicles, of trucks," he says.

The area is still facing a significant threat because more rain could spur new landslides, Rengers says. And it doesn't take very much rain for that to happen.

"In that area, our prediction was about a half an inch in an hour could cause a debris flow," he says. "And what we saw in Montecito, one of the rain gauges showed half an inch in five minutes."

The USGS studies statistics of past landslides to predict how much rainfall will cause future natural disasters. The agency conducted risk assessments for each of last year's fires in the state.

Mudslides have been a fact of life in Southern California for decades. The National Weather Service's list of floods, mudslides, debris flows and landslides in the area is over 60 pages long. After the La Conchita landslide that killed 10 people in Jan. 2005, the USGS said historical evidence showed that equally destructive landslides would keep happening.

"Historical accounts and geologic evidence show that landsliding of a variety of types and scales has been occurring at and near La Conchita for many thousands of years, and on a relatively frequent basis, up until the present," the USGS wrote in the report. "There is no reason to believe this pattern of landsliding will stop."

Rengers says residents should always listen to evacuation orders because it's impossible to outrun a mudslide.

"The main thing for future safety is that people just heed evacuation warnings because they sound like a freight train, and they move just as quick," Rengers says. "You're not going to get out of the way if you see one coming."